CHEST RT is investigating the safety and effectiveness of combining chemotherapy and immunotherapy with chest radiation therapy for the treatment of extensive stage small cell lung cancer (ES-SCLC). For over 20 years, a combination of chemotherapy using etoposide with either cisplatin or carboplatin had been used to treat ES-SCLC. Adding immunotherapy to the chemotherapy combination has been shown to help boosts the body’s natural defences to fight cancer, improving response to treatment. This combination of chemotherapy with immunotherapy is now the standard of care treatment. Research has shown that radiation therapy also improves the ability of the immune system to recognise tumours. The researchers would like to investigate whether combining radiation therapy with the standard chemo/immunotherapy may further improve patients response to treatment.